The primary objective of this research is to further our understanding of the mechanics of the healthy cochlea. A secondary objective is to understand how sound is transmitted through the healthy middle ear. To attain these objectives we use two powerful experimental techniques: intracochlear pressure measurements and interferometric measurements of intracochlear and middle ear motion. Specific experiments measure sound transmission within the cochlea to explore the interaction between the cochlea's active mechanics and the cochlear traveling wave, and sound transmission between the cochlea and the ear canal to determine how the middle ear filters cochlear emissions. Other experiments will measure detailed motion of the cochlea's basilar membrane; subtle spatial variations could reveal the operation of the cell- based forces that shape cochlear tuning. In order to understand the passive substrate for cochlear tuning, upon which the interesting "active" cell-based forces must build, one experiment will measure the stiffness and resistance of the cochlea's basilar membrane and organ of Corti in situ, another will measure tuning in cochleae in which the organ of Corti was ototoxically damaged. Finally, one experiment will trace the source of the observed middle ear transmission delay. The measurements are aimed at understanding the mechanics of the normal cochlea and middle ear. Nevertheless, they are significant to the treatment of the hearing impaired. While cochlear implants have been breathtakingly successful, a divide remains between the functional hearing available with an implant and the natural auditory experience of music, language, and nature. To decrease this divide, cochlear implants and hearing aids continue to be improved and a better understanding of the natural processing of the cochlea will benefit this work. Beyond implants and aids, the promise of repairing the organ of Corti through therapies that regenerate or repair cochlear hair cells is very exciting, and the understanding of the normal mechanics will help guide these advances. In the case of the middle ear, surgical therapies are available but are not always successful. Better understanding the transmission of sound through the middle ear is one of the goals of this proposal, and gaining that knowledge could influence its surgical repair.